1. Field of the Invention
The present invention relates to a method for a channel of a Code Division Multiple Access telecommunication system, said Code Division Multiple Access telecommunication system implementing a phase of communicating data conveyed by a plurality of transport channels, said Code Division Multiple Access telecommunication system comprising at least one sending entity and at least one receiving entity for communicating said data, said phase of communicating data comprising a plurality of rate matching steps, each of said plurality of rate matching steps executing a transformation of an input block of an initial size into an output block of a final size by puncturing or repeating at least one bit of said input block.
2. Discussion of the Background
The 3GPP (3rd Generation Partnership Project) group is an association whose members originate from several regional standardization bodies including in particular the ETSI (European Telecommunication Standardization Institute) and the ARIB (Association of Radio Industries and Businesses). Its object is the standardization of a third-generation telecommunication system for mobiles. One of the fundamental aspects distinguishing third-generation from second-generation systems is that, apart from the fact that they will use the radio spectrum more efficiently, they will allow very great flexibility of service. Second-generation systems offer an optimized radio interface for certain services. For example GSM (Global System for Mobiles) is optimized for the transmission of speech (telephony). Third-generation systems will offer a radio interface adapted for all kinds of services and combinations of services.
One of the issues at stake with third-generation mobile radio systems is that of efficiently multiplexing, on the radio interface, services which do not have the same demands in teams of quality of service (QoS). Quality of service is defined, conventionally, according to at least one criterion comprising in particular a processing delay, a bit error rate and/or an error rate per transported block. These different qualities of service require corresponding transport channels having different channel codings and channel interleavings. Moreover, they demand different maximum bit error rates (BER). For a given channel coding, the demand with regard to the BER is satisfied when the coded bits have at least a certain coding-dependent ratio Eb/I. The ratio Eb/I expresses the ratio of the average energy of each coded bit to the average energy of the interference.
It follows that the different qualities of service do not have the same demand in terms of the ratio Eb/I. Now, in a system of the CDMA (Code Division Multiple Access) type, the capacity of the system is limited by the level of interference. It is therefore necessary to fix the ratio Eb/I as correctly as possible for each service. Therefore, a rate matching operation, for balancing the ratio Eb/I is necessary between the various services. Without this operation the ratio Eb/I would be fixed by the service having the greatest demand, and as a result the other services would have “too good” a quality, thereby impacting directly on the capacity of the system.
This raises a problem since it is necessary in some manner that the rate matching ratios be defined identically at the two ends of the radio link.
The present invention relates to a configuring method for defining rate matching ratios identically at the two ends of a CDMA radio link.
In the OSI model (Open System Interconnection) from the ISO (International Standardization Organization), a telecommunication equipment is modelled by a layered model constituting a stack of protocols where each level is a protocol supplying a service to the level above. Level 1 is in particular responsible for implementing channel coding and channel interleaving. The service supplied by level 1 is referred to as “transport channels”. A transport channel allows the higher level to transmit data with a certain quality of service. The quality of service is in particular characterized by the delay and the BER.
In order to satisfy the quality of service demand, level 1 uses a certain encoding and a suitable channel interleaving.
The known solutions, and in particular those proposed in the 3GPP project, will be described with regard to FIGS. 1 and 2.
FIG. 1 is a diagrammatic view illustrating the multiplexing of the transport channels on the uplink in the current 3GPP proposal;
FIG. 2 is a diagrammatic view illustrating the multiplexing of the transport channels on the downlink in the current 3GPP proposal.
Represented in FIGS. 1 and 2 are the block diagrams for interleaving and multiplexing as defined by the current proposal by the 3GPP group, although this proposal has not yet been finalized.
In these figures, similar blocks bear the same numbers. In both cases the uplink (from the mobile station to the network) may be distinguished from the downlink (from the network to the mobile station), and only the transmission part is represented.
Each transport channel, labelled 100, periodically receives a transport blocks set from an higher level, labelled 102. The number of transport blocks 100 in this set, as well as their sizes, depend on the transport channel. The minimum period at which the transport blocks set is supplied corresponds to the time span of the interleaving of the transport channel. The transport channels with one and the same quality of service (QoS) are processed by one and the same processing chain 103A, 103B.
In each of the processing chains 103A, 103B, the transport channels, in particular after channel encoding and channel interleaving, are multiplexed together by concatenation in step 104. This multiplexing is carried out per multiplexing frame. A multiplexing frame is the smallest unit of data for which demultiplexing may be carried out at least partially. A multiplexing frame typically corresponds to a radio frame. The radio frames form consecutive time intervals synchronized with the network, and numbered by the network. In the proposal by the 3GPP group, a radio frame corresponds to a duration of 10 ms.
The 3GPP proposal comprises the service-specific coding and interleaving option represented diagrammatically at 103C. The possibility of such an option is being considered at present since its indispensability or otherwise has not yet been determined.
In the general case, a processing chain 100A firstly comprises a step 106 during which a bit word termed the FCS (Frame Check Sequence) is attached to each transport block. The bit word FCS is typically calculated by the so-called CRC technique (Cyclic Redundancy Check) which consists in considering the bits of the transport block to be the coefficients of a polynomial P and in calculating the CRC from the remainder of the polynomial (P+P0) after dividing by a so-called generating polynomial G, where P0 is a predefined polynomial for a given degree of P. The attachment of the bit word FCS is optional, and certain transport channels do not include this step. The exact technique for calculating the bit word FCS also depends on the transport channel, and especially on the maximum size of the transport blocks. The usefulness of the bit word FCS is in detecting whether the transport block received is valid or corrupted.
The next step 108 consists in multiplexing together the transport channels (TrCH) of like quality of service (QoS). This is because those transport channels which have the same quality of service may use the same channel coding. Typically, the multiplexing at 108 is carried out by concatenating the transport blocks sets with their FCS for each transport channel.
The next step, labelled 110, consists in performing the channel encoding. On exit from the channel encoder 110 there is a set of coded blocks. Typically, in the case of a convolutional code, we have either zero or a single coded block of variable length. The length is given by the formula:Noutput=Ninput/(coding rate)+Ntail(length of the coded block).with:                Noutput=number of bits at output (length of the coded block);        Ninput=number of bits at input;        coding rate=constant ratio; and        Ntail=fixed quantity of information, independent of Ninput, serving to empty the channel decoder cleanly at the time the coded block is received.        
It is onwards of this step 110 that the uplink differs from the downlink.
In each transport channel, whether the uplink (FIG. 1) or the downlink (FIG. 2), a rate matching step is implemented after the channel encoding step 110. This step is labelled 112 for the uplink and 114 for the downlink. Rate matching is not necessarily performed immediately after channel encoding 110.
The objective of the rate matching step 112 or 114 is to balance the ratio Eb/I between the transport channels with different qualities of service. The ratio Eb/I gives the average energy of a bit with respect to the average energy of the interference. In a system using multiple access CDMA technology, the greater this ratio the greater is the quality which may be obtained. It will be understood therefore that transport channels having different qualities of service do not have the same need in terms of Eb/I, and that in the absence of rate matching, certain transport channels would have “too” good a quality of service relative to their respective needs, fixed as it is by the most demanding channel in terms of quality of service. Such transport channels would then needlessly cause interference. Rate matching therefore has a role of matching the Eb/I ratio. Rate matching is such that X bits at input give Y bits at output, thus multiplying Eb/I by the ratio Y/X, hence the matching capability. In what follows, the ratio Y/X is referred to as the rate matching ratio, also known as the rate matching ratio.
Rate matching is not done in the same way in the uplink and in the downlink.
This is because, in the uplink, it has been decided to transmit continuously, since discontinuous transmission worsens the peak/average ratio of the radio-frequency power at the output of the mobile station. The closer this ratio is to I the better. This is because, if this ratio is worsened (that is to say increased), this signifies that the power amplifier requires a greater margin (backoff) of linearity with respect to the mean operating point. On account of such a margin, the power amplifier would be less efficient and would therefore consume more for the same average power emitted, and this would in particular unacceptably reduce the mobile station's battery-powered endurance. Because it is necessary to transmit continuously on the uplink, the rate matching ratio Y/X cannot be constant. This is because the sum Y1+Y2+ . . . Yk of the numbers of bits after matching must be equal to the total number of bits in the radio frame for the data. This number may take only certain predefined values N1, N2, . . . , Np. It is therefore appropriate to solve the following system in k unknowns Y1, . . . , Yk:
  {                                          ∀                                                  ⁢                          i              ∈                                                {                                      1                    ,                    …                    ⁢                                                                                  ,                    k                                    }                                ⁢                                  Y                  i                                                              =                      λ            ·                          (                                                Eb                  i                                /                I                            )                        ·                          X              i                                                                                              Y              1                        +            …            +                          Y              k                                =                      N            j                                                        ∀                                          ⁢                      i            ∈                                                            {                                      1                    ,                    …                    ⁢                                                                                  ,                    k                                    }                                ⁢                λ                            ≥                                                1                  -                                      P                    i                                                                                        Eb                    i                                    /                  I                                                                        where Xi and Ebi/I and Pi are characteristic constants of each transport channel, and where it is sought to minimize Nj from among the p possible values N1, N2, . . . , Np (note: Pi is the maximum allowable puncture rate for a coded transport channel).
Thus, in the uplink, the rate matching ratios Y/X for each transport channel are not constant from one multiplexing frame to the next, but are defined to within a multiplicative constant: the pairwise ratios between these ratios therefore remain constant.
In the downlink, the peak/average ratio of the radio-frequency power is in any case very poor since the network transmits to several users simultaneously. The signals destined for these users combine constructively or destructively, thereby inducing wide variations in radio-frequency power emitted by the network, and hence a poor peak/average ratio. It was therefore decided that for the downlink the balancing of Eb/I between the various transport channels would be done with a rate matching having a constant rate matching ratio Y/X, and that the multiplexing frames would be supplemented with dummy bits, that is to say bits which are not transmitted, that is to say discontinuous transmission.
Thus, the difference between the uplink and the downlink lies in the fact that in the uplink the rate matching 112 is dynamic so as to supplement the multiplexing frames, whereas in the downlink the rate matching 114 is static and the multiplexing frames are supplemented through the insertion of dummy bits in the immediately following step 124.
The rate matching, whether dynamic or static, is done either by repetition or by puncturing, according to an algorithm which was proposed to the ETSI by the Siemens company (registered trade mark) in the technical document referenced SMG2/UMTS-L1/Tdoc428/98. This algorithm makes it possible to obtain non-integer puncture/repetition ratios, and it is given in Table 1 for information.
Input data:Xi -number of bits at inputYi -number of bits at outputNpr=|Yi-Xi| -number of bits to be repeated or to be punctured(if Y1 > Xi, we repeat, otherwise we puncture)The puncturing/repetition rule is as follows:e=2*Np/r − Xi - -initial error between the current and desiredpuncture/repetition ratiosx = 0 - -index of the current bitwhile x < Xi doif e > 0 then - - test whether bit number xshould be repeated/punctured {open oversize bracket} {puncture or repeat bit number xe = e + (2*Np/r − 2 * Xi) - -update the errorelse        e = e + 2*Np/r - -update the error  end if  x = x + 1 - -next bit- end_do
The particular feature of this algorithm is that, when it operates in puncture mode, it avoids the puncturing of consecutive bits, but on the contrary tends to maximize the spacing between two punctured bits. As far as repetition is concerned, the repetition bits follow the bits which they repeat. Under these conditions, it will be understood that it is beneficial for the rate matching to be done before interleaving. This is because, for repetition, the fact that an interleaving follows the rate matching makes it possible to space the repeated bits apart. For puncturing, the fact that an interleaver precedes the rate matching gives rise to the risk that the rate matching might puncture consecutive bits on exit from the channel encoder.
It is therefore advantageous for the rate matching to be done as high up as possible, that is to say as close as possible to the channel encoder.
Moreover, each processing chain 103A, 103B also comprises, after the channel encoding step 110, a first interleaver labelled 116 for the uplink and 118 for the downlink, followed by a step of segmentation per multiplexing frame labelled 120 for the uplink and 122 for the downlink. The first interleaver 118 is not necessarily located immediately after the channel encoding 110.
For the downlink, it is possible to place the rate matching 114 right at the output of the channel encoding 110, since the rate matching ratio is constant. Hence, a priori only a single interleaver 118 is needed.
However, a second interleaver 136 is necessary, since the multiplexing of the transport channels of different qualities of service QoS is done by straight-forward concatenation, and since such a method would in fact limit the time span of each multiplexed block.
For the uplink the rate matching ratio may vary with each multiplexing frame. This explains the need for at least the first interleaver 116 before the rate matching 112 so as to distribute the bits of the coded block over several multiplexing frames, and for a second interleaver 128 placed after the rate matching so as to space apart the bits repeated by the rate matching 112.
Thus in the block diagrams of FIGS. 1 and 2 may be seen two interleavers referred to in the block diagrams as the first and second interleavers. The first interleaver 116, 118 is an interleaver whose time span is equal to the interleaving time span for the corresponding transport channel. This span may be longer than the duration of a multiplexing frame and is typically a multiple thereof in a constant ratio. This is why this first interleaver 116, 118 is also sometimes referred to as an inter-frame interleaver.
The second interleaver 126, 128 is also referred to as an intea-frame interleaver since its time span is that of a multiplexing frame.
Consequently the step of segmentation per multiplexing frame labelled 120, 122 is situated between the first 116, 118 and the second 128, 126 interleavers (when there is a second interleaver). This step consists in segmenting the blocks which are coded and are interleaved by the first interleaver into as many segments as is equal to the ratio of the time span of the first interleaver to the duration of a multiplexing frame. This segmentation is typically done in such a way that the concatenation of the segments once again yields the interleaved coded block.
It will be noted that, in the uplink, this segmentation step 120 is necessarily located before the rate matching 112. This is because the rate matching 112 is done according to a ratio established dynamically multiplexing frame by multiplexing frame, and it is not therefore possible to do it on a unit of data which may extend over several multiplexing frames.
In the uplink and the downlink, a step 130 of segmentation into physical channels is implemented before each second interleaver 126, 128. Likewise, the second interleavers 126, 128 are followed by a step 132 of physical channel mapping for transmission proper.
At present, only the multiplexing, channel encoding, interleaving and rate matching algorithms are defined and discussed. There is no rule making it possible to fix the way in which with a size X of a block input into the bit rate matcher there is associated a size Y of the block obtained on output. We are reduced to assuming that all the combinations of the pairs (X, Y) are predefined and saved in memory in a frozen manner. Only one of the following two things is possible:                either the set of pairs (X, Y) remains frozen and no flexibility of definition of this set of pairs (X, Y) for the service concerned is obtained, which is contrary to the sought-after effect;        or the set of pairs (X, Y) is negotiated between the mobile stations and the telecommunication network involved and a high number of signalling bits and hence additional immobilization of resources has to be envisaged.        
A rule for determining the size Y of a rate matched block which is rate matched with the other blocks, on the basis of the size X of this block before rate matching is necessary at least in the uplink. This is because, since the services have variable bit rates, the number of transport blocks provided for each transport channel is variable. The list (X1, X2, . . . , Xk) of the sizes of blocks to be rate matched may consequently vary from multiplexing frame to multiplexing frame. Neither is the number k of elements in this list necessarily constant.
As the size Yi associated with the size Xi does not depend only on Xi but on the entire list (X1, X2, . . . , Xk) owing to the dynamic matching, it follows that there exists a list (Y1, Y2, . . . , Yk) for each list (X1, X2, . . . , Xk). The number of lists may therefore be very large, at least as large as the number of combinations of transport formats. A transport format combination is a quantity defining how to demultiplex the multiplexing frame.
Thus, the sending and receiving entities should employ the same association list (X1, X2, . . . , Xk) (Y1, Y2, . . . , Yk). The signalling of this list of association between these two entities at the time of connection of the composite of coded transport channels represents a non-negligible cost in terms of signalling bits. A composite of coded transport channels includes at least two groups of coded transport channels. Moreover, it would then be necessary to provide for the exchange of a new list of associations (X1, X2, . . . , Xk)→(Y1, Y2, . . . , Yk) with each addition or removal included within the composite of coded transport channels.
Moreover, the exact matching of the ratio Eb/I depends on the technology of the channel decoder for each quality of service QoS. The performance of such a device can vary from one manufacturer to another depending on their respective know-how. In fact, this rate matching does not depend on the absolute performance of each decoder, but on their performance relative to one another, which may therefore vary from one manufacturer to another, if the performance of one of them varies.
It is not therefore possible for the sending and receiving entities employed to be able to “negotiate” the matching of the ratios (Eb/I) through an appropriate exchange of signalling messages.
To explain this, let us imagine two qualities of service A and B, and two manufacturers M and N, M and N have the same channel decoder for A, but M has a far more efficient decoder than that of N for B. It is then clear that manufacturer M could benefit from a smaller ratio Eb/I for B as this would decrease the total power required and would therefore produce a gain in capacity which would enable M to sell more mobile telecommunication equipments to network operators by arguing thus.
It would therefore be very useful to be able to signal parameters making it possible to define the rule X→Y for determining the size Y of a block after rate matching from the size X of the block before matching. This would make it possible to negotiate or to re-negotiate the proportions of the ratios Eb/I. This signalling must be as inexpensive as possible.
This adjustment during connection of the ratios Eb/I performed by the higher levels therefore signifies that if two telecommunication stations A and B wish to establish or modify a connection over which there is a service multiplexing, then they follow the following steps:
1. B signals to A what is the maximum load N of a multiplexing frame B can send.
2. A determines the ideal proportion for A of the ratios Eb/I from:                the value of N received from B        the maximum puncture rate allowed by A for each quality of service QoS,        the relative demands for each quality of service QoS in terms of Eb/I        the minimum performance demand specified for A.        
3. A signals to B what proportion of the ratios Eb/I A expects.
Step 1 is not necessarily present. Systems may be imagined in which the maximum load is known in advance and forms part of the characteristics of the system. That said, such a system would be highly improbable in view of its lack of flexibility.
It may happen that the proportion of the ratios Eb/I which is determined by A is sub-optimal in relation to the sought-after aim which is that no transport channel should have more than it deserves. This is a compromise situation in which it is preferred to reduce the capacity of the network provided that the connection of the combination of services can be established.
Such a compromise is acceptable insofar as the degradation is within the limits fixed by the minimum performance demand defined in the system specification.
It may also happen that the actual tolerance limit is partly at the discretion of the network. This would make it possible to define non-guaranteed levels of service, in which the service is provided when the traffic conditions so permit, and otherwise it is re-negotiated downwards.
There will certainly be a specification of possible combinations of services. In this specification, for each combination of services, there will be associated a set of combinations of transport formats. This will definitely be the case for the basic services such as the conventional telephony service, and all the associated services such as call signalling, standby, etc.
However, the number of potential combinations may well increase in the future, and clear rules will then be needed in order for the higher levels to determine what combinations are possible, how to negotiate them, and/or to renegotiate them, and also in order for them to determine the set of transport format combinations for a given combination.
The higher levels ought therefore to be able with the aid of simple arithmetic algorithms to determine which combinations of transport formats are possible. To do this, there are at least three arithmetic rules which the higher levels should apply:                The first rule, concerning channel encoding, makes it possible to convert the number of elements of the transport blocks sets and their respective sizes into the number of elements of the sets of coded blocks and their respective sizes. For example, this rule may be of the type:        
Y=X/(coding rate)+Ntail, where “coding rate” and “Ntail” are characteristic constants of the code.                The second rule concerning segmentation converts the size of a coded block into the size of a segment produced by the segmentation per multiplexing frame. In general this rule is a simple division by F when the transmission interval of the associated transport channel corresponds to F multiplexing frames. However, it is not yet clear whether the segmentation is equal or unequal. In the case of equal segmentation, the coded blocks have a size which is a multiple of F. In this case, all the segments are of the same size since there is no rounding error when dividing by F. In the case of unequal segmentation, the size of the segment is defined to within 1 bit, on account of the rounding-up or rounding-down error, and the serial number of the segment must be known in order to reduce the ambiguity. For example, if 80 bits are to be segmented into F equal to 8 frames, then all the segments will contain 10 bits and there is no need to know the serial number of the segment (or position of the segment) concerned in order to ascertain its size. On the other hand, if 78 bits are to be segmented into F equal to 8 frames, then 6 segments will contain 8 bits and two other segments will contain 9 bits and it is necessary to know the serial number of the segment in order to ascertain its size.        The third rule is that which makes it possible to deduce, from the size X of a block to be rate matched, the size Y of the rate matched block.        
This third rule is not specified and the invention solves this problem of deducing the corresponding sizes for the blocks to be matched.